This invention relates to star block copolymers of vinyl aromatic hydrocarbons and/or conjugated dienes and polydimethylsiloxane. The invention also relates to a process for making such block copolymers.
Linear block copolymers of polystyrene and polydimethylsiloxane have been synthesized, both by graft and block copolymerization. In block copolymerization of such linear polymers, polystyrene is produced by anionic polymerization with an organo lithium promoter and the living polymer (PS.sup.- Li.sup.+) created thereby is reacted with hexamethylcyclotrisiloxane, (Me.sub.2 Sio).sub.3, in the presence of a polar promoter wherein a block of polydimethylsiloxane grows on the end of the living vinyl aromatic hydrocarbon polymer block.
Such polymers incorporate the strength of vinyl aromatic hydrocarbons such as polystyrene and the high flexibility of polydimethylsiloxane and its extremely low solubility parameter and Tg compared with polystyrene. These polymers are useful for impact modification of engineering thermoplastics and surface-inactive coatings. The present invention produces star block copolymers of vinyl aromatic hydrocarbons and/or conjugated dienes and polydimethylsiloxane which can be used for similar purposes but have much lower viscosity than linear analogs of comparable molecular weight. As such, they can be manufactured at higher solids level in solution (and thus are cheaper) and they will process more easily for end use applications.